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Revision as of 09:51, 29 January 2008 by leisti (Talk | contribs)

Getting started with Java ME

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(This page is very much work in progress. Please add more information, screenshots, and links to other relevant pages.)


Getting your first Java ME application (MIDlet) running on a real device requires a number of steps. This Wiki page is a guide through that process. As it is targeted at first-time MIDlet developers, some things are simplified, and more detailed information can be found elsewhere.


Contents

Is Java ME the right tool for your requirements?

Java ME is a limited subset of the standard Java (Java SE) available on the desktop computers, with some additional mobile phone-related APIs. There are a number of limitations you need to keep in mind:

  • MIDlets run in a sandbox because of security reasons. (There are confirmations when using certain functionalities, like networking or sending SMS messages.)
  • There is no JNI (Java Native Interface) so you cannot extend the capabilities of Java ME environment on the phone.
  • There are no Swing or AWT classes. MIDlets use their own (simplistic) UI classes.
  • The capabilities of the Java ME environment vary widely, meaning that the phones have different set of optional APIs implemented (examples include access to files, access to phonebook, video/audio recording, 3D graphics, etc.)

So first of all, before starting anything else, think whether Java ME is the right choice for you, or whether you should instead use Flash Lite for your application (runs both on Series 40 and S60 phones), or whether you should program in native Symbian C++ (for S60 phones only). (However, native C++ development for SymbianOS is quite tedious and error-prone, so just for productivity reasons, Java ME might be a better choice.)

If Java ME seems to be the way to go, you can find a lot of good information in this Wiki or in Forum Nokia's Java Discussion Forums. There is also documentation available elsewhere on the Forum Nokia web site, as well at Sun Web site and IBM DeveloperWorks Web site.


Overview of the development process

First note that you have to code, compile and package the MIDlet on your development computer. You cannot create a MIDlet on the phone itself (yet).

To write MIDlet code, use whatever text editor or IDE (Integrated Development Environment) you prefer. (But see the Nokia-supported IDEs below.) Then compile the code with the javac compiler which is comes with a Java SE SDK (JDK). Even though you do not need to worry about it right now, your code is also preverified at this stage.

Before being able to install the MIDlet on a real device, you still need to package the MIDlet classes and resources to a JAR (Java Archive) file and generate a JAD (Java Application Descriptor) file. The JAR file contains all the code needed to run the application. The JAD file is a text file containg descriptive information about the MIDlet (size, needed APIs, MIDlet name, Vendor name, optional signing information), which the phone (and user) can use to decide whether or not to download the MIDlet at all. You need only to install the JAR file on the phone to be able to run the MIDlet; however, it is strongly recommended to use both of the files to install the application.

At this stage, you can test the MIDlet on your development workstation using the phone emulator, and then most probably make changes, then recompile and repackage it. When you are satisfied with the result, you can transfer the generated JAR and JAD files to a real phone, install the app, and test it. (You should always test a MIDlet on a real device before distributing it.)

You can also use some online resources to test your application on real devices, such Remote Device Access (RDA) or Device Anywhere.


Required hardware

Because of the requirements of Nokia's development tools, your development computer has to be a PC running Microsoft Windows 2000 (SP 4) or Microsoft Windows XP (SP 2).


Required software for S60

The combination of tools listed here (all free) has been tested by the writer (note: not yet!) to work for developing applications for S60 using Java ME. For future editors of this article: please make sure that your edits have been tested in practice, to prevent developer frustration.

tool recommended version download notes
a Java SE (Standard Edition) Development Kit (JDK) JDK 6 Update 4 (note: Java* 6 was formerly known as J2* 1.6) here (search for "JDK 6 Update 4") Select the Windows platform, then select the "Windows Offline Installation" option (or if you're feeling more adventurous, the "Windows Online Installation" option). Save the installation file, then run it. The installation directory is C:\Program Files\Java\jdk1.6.0_04. Do not move the installation elsewhere, because other tools look for Java installations in this directory.
the Sun Java Wireless Toolkit for CLDC 2.5.2 here (search for "Sun Java Wireless Toolkit 2.5.2 for CLDC") The installation program should automatically detect that you have JDK 6 Update 4 installed in directory C:\Program Files\Java\jdk1.6.0_04. If not, browse for that directory at the "Java (TM) Virtual Machine Location" phase of the installation. The default installation directory is C:\WTK2.5.2; change it if you want to.
Nokia SymbianOS/S60 SDK for Java (JDK) S60 3rd edition, Feature Pack 2, MIDP SDK Beta here
the Eclipse IDE 3.2.2 here The default installation directory is C:\Program Files\eclipse. The writer prefers to append the version number to the installation; i.e., C:\Program Files\eclipse-3.2.2.
the EclipseME plugin for Eclipse 1.7.7 here This is the last EclipseME version that runs on top of Eclipse v3.2. Follow the installation instructions here.
Java ME Developer's Library 2.1 here (search for "Java ME Developer's Library v2.1 (Eclipse plug-in)") This documentation is not absolutely necessary for development, but it is very useful. After you've installed it, it is available to you from Eclipse's Help function.


Note that in addition to Eclipse, Nokia also supports the NetBeans IDE (plus its Mobility Pack) for S60/Java ME development. See here for more details.

Emulators and SDKs for Series 40 and Series 80

You should also download and test your application in other vendors' emulators, too. (Motorola, Sony Ericsson, Samsung, etc.)

As for S60 development, use a PC with Windows 2000 or Windows XP. Use the NetBeans or Eclipse IDEs.


Coding and compiling your first MIDlet

See Hello World in Java ME

Typical pitfalls

Compiling the Java ME code with wrong Java compliance option

The correct compliance is 1.3 or 1.4. Compiling with 5.0 or 6.0 compliance makes it impossible to install the MIDlet.

Compiling and packaging the MIDlet for incorrect CLDC or MIDP version

This is especially true when using Sun Wireless Toolkit, which by default builds the MIDlet for MIDP 2.1. However, most of the current phones have only MIDP 2.0 available. (Note, that you can install on a phone a MIDlet with lower CLDC/MIDP version than what the phone supports, but you cannot install a MIDlet with higher CLDC/MIDP version on the phone. Check the supported CLDC and MIDP versions on FN device specification pages.)

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